Tetradecamethylhexasiloxane

Biodegradation in water: screening tests

Link to relevant study record(s)

Reference

Endpoint:

biodegradation in water: ready biodegradability

Type of information:

experimental study

Adequacy of study:

key study

Reliability:

1 (reliable without restriction)

Rationale for reliability incl. deficiencies:

guideline study

Qualifier:

according to guideline

Guideline:

OECD Guideline 310 (Ready Biodegradability - CO2 in Sealed Vessels (Headspace Test)

GLP compliance:

yes

Oxygen conditions:

aerobic

Inoculum or test system:

activated sludge, domestic, non-adapted

Details on inoculum:

- Source of inoculum/activated sludge (e.g. location, sampling depth, contamination history, procedure): Activated sludge was collected from the Cambridge Wastewater Treatment Facility, Cambridge, Maryland on March 30, 2009. The Cambridge facility treats predominantly residential wastes.

- Preparation of inoculum for exposure: The sludge was sieved using a 2-mm screen and then aerated at test temperature until its adjustment. The activated sludge was diluted in test medium to approximately 30 mg total suspended solids/L and aerated with CO2-free air until its adjustment. The inoculated medium used in the test was prepared by diluting the preconditioned activated sludge in test medium to 4 mg total suspended solids/L.

Duration of test (contact time):

28 d

Initial conc.:

20 mg/L

Based on:

DOC

Parameter followed for biodegradation estimation:

CO2 evolution

Details on study design:

TEST SYSTEM
- Culturing apparatus: glass serum bottles with a nominal volume of 160 mL. The chambers were sealed with butyl rubber septa and crimp caps.


SAMPLING
- Sampling frequency: Replicate chambers from the control, reference and treatment groups were removed for IC analysis once a week over the test period. Replicate chambers from the toxicity control group were removed for IC analysis on Days 1 and 28. Three replicate test chambers from each group were analyzed at each respective sacrifice interval, except for the last interval (5 replicates each).

- Sampling method: Samples for analysis were prepared by converting CO2 to carbonate. One milliliter of 7M NaOH was injected into the test chambers, which were subsequently shaken for at least 1 hour at test temperature. After mixing, the chambers were removed from the shaker table, allowed to settle and then samples were removed for IC analysis.

Reference substance:

benzoic acid, sodium salt

Parameter:

% degradation (CO2 evolution)

Value:

0

Sampling time:

28 d

Details on results:

Test substance: -3.7% in 28 days
Reference substance: 96.5% in 28d (>60% in 7 days)
Toxicity control: 46.2% in 28d

Table 1: Inorganic carbon concentration (mg C/l) and calculated % biodegradation for inoculum blank, reference substance, test substance and toxicity control

Type of suspension	1d		7d		14d		21d		28d
	Inorg. C (mgC/l)	(%) biodeg	Inorg. C (mgC/l)	(%) biodeg	Inorg. C (mgC/l)	(%) biodeg	Inorg. C (mgC/l)	(%) biodeg	Inorg. C (mgC/l)	(%) biodeg	Cumulative average (%) biodeg (Day 28)
Inoculum Blank	-	-	0.3; 0.1; 0.4	-	2.3; 2.4; 1.8	-	2.9; 2.8; 3.1	-	2.8; 2.8; 3.1; 2.6; 3.1	-	-
Reference substance	-	-	16.9; 16.8; 17.4	83.8; 83.4; 86.1	18.8; 19.5; 19.1	84.1; 87.4; 85.4	20.5; 20.4; 20.4	88.4; 88.2; 88.2	22.1; 22.0; 22.2; 21.9; 21.9	96.7; 96.3; 97.5; 96.1; 95.8	96.5
Test substance	-	-	0.1; 0.0; 0.0	-0.7; -1.3; -1.5	1.9; 2.3; 1.5	-1.5; 0.4; -3.2	2.4; 2.3; 2.4	-2.9; -3.2; -2.6	2.2; 2.2; 2.4; 2.1; 2.0	-3.5; -3.7; -2.6; -3.9; -4.6	-3.7
Toxicity control	13.4; 13.6; 13.6	33.7; 34.3; 34.4	-	-	-	-	-	-	21.3; 21.1; 21.1; 21.4; 20.9	46.6; 46.0; 45.9; 46.8; 45.5	46.2

 

Validity criteria fulfilled:

yes

Interpretation of results:

under test conditions no biodegradation observed

Conclusions:

0% biodegradation in 28 days was determined in a reliable study conducted according to an appropriate test protocol, and in compliance with GLP.

Description of key information

Biodegradation in water: screening tests: 0% in 28 days (OECD 310), read-across from a structurally-related substance.

Key value for chemical safety assessment

Biodegradation in water:

under test conditions no biodegradation observed

Additional information

There are no reliable ready biodegradation data available for tetradecamethylhexasiloxane (L6), therefore good quality data for the structurally-related substance, octamethyltrisiloxane (L3, CAS No. 107-51-7), have been read across. L6 and L3 are structurally-similar substances, both are linear siloxanes.

L3 is a linear siloxane chain with three silicon atoms, connected by two oxygen atoms. All silicon atoms present are fully substituted with methyl groups. L6 is a structurally related linear siloxane, with six silicon atoms and five oxygen atoms.

 

L3 and L6 are within a group of substances within which, in general, there is no evidence of any significant biodegradation.

The table below presents ready biodegradation data for substances within the group of siloxanes (alkyl, vinyl, aryl or hydrogen substituted).

It is therefore considered valid to read-across the results for L3 to fill the data gap for the registered substance. Additional information is given in a supporting report (PFA, 2017at) attached in Section 13.

Table 4.5: Ready biodegradation data for substances within the siloxane group

CAS	Name	Readily biodegradable? Yes/no	Result: Biodegradation after 28 days (%)*	Guideline	Test type	Reliability
107-46-0	Hexamethyldisiloxane	No	2%	OECD 301C	BOD	1
107-51-7	Octamethyltrisiloxane	No	-3.70%	OECD 310	CO2	1
540-97-6	Dodecamethylcyclohexasiloxane	No	4.5%	OECD 310	CO2	1
541-02-6	Decamethylcyclopentasiloxane	No	0.14%	OECD 310	CO2	1
541-05-9	Hexamethylcyclotrisiloxane	No	0.06%	OECD 310	CO2	1
556-67-2	Octamethylcyclotetrasiloxane	No	3.70%	OECD 310	CO2	1
1873-88-7	1,1,1,3,5,5,5-Heptamethyltrisiloxane	No	0%	OECD 310/ ISO Guideline No 14593	CO2	1
2627-95-4	1,1,3,3-Tetramethyl-1,3-divinyldisiloxane	No	0%	OECD 301D	BOD	1
17928-28-8	1,1,1,3,5,5,5-Heptamethyl- 3-[(trimethylsilyl)oxy] trisiloxane	No	0%	OECD 301F	BOD	1
17955-88-3	1,1,1,3,5,5,5-Heptamethyl-3-octyltrisiloxane	No	10 – 32 % after 25 d	OECD 301B	CO2	1
60111-47-9	3-[(Dimethylvinylsilyl)oxy]-1,1,5,5-tetramethyl-3-phenyl-1,5-divinyltrisiloxane	No	2.20%	OECD 301F	BOD	1
69304-37-6	1,3-Dichloro-1,1,3,3-tetraisopropyldisiloxane	No	0% (BOD); 100% (GC)	OECD 301C	BOD	1
70940-00-0	1,1,7,7,-Tetramethyl-3,3,5,5,-Tetraphenyl-Tetrasiloxane	No	9.7 and 9.9%	OECD 301B	CO2	1
180073-46-5	Pentasiloxane, 1,1,9,9-tetramethyl-3,3,5,5,7,7-hexaphenyl-	No	5.2 and 4.4%	OECD 301B	CO2	1
352230-22-9	Siloxanes and Silicones, di-Ph, Ph (trimethylsilyl)oxy	No	0%	OECD 301F	BOD	1

In the study with L3, 0% biodegradation was observed in 28 days.

The study was conducted according to OECD 310 and is considered reliable. This is selected as the key study.

References: PFA. (2017at): Siloxane Category Report for Environmental Endpoints. Owner company: Reconsile. Report date: 2017-06-02